Regulation mechanism of heat input on in situ tempering and precipitation behavior of wire arc additively manufactured 1,600 MPa low-alloy ultra-high-strength steel

Abstract

With surging demand for lightweight, high-strength metal components in high-end equipment and large-scale engineering structures, additive manufacturing of low-alloy ultra-high-strength steel (LUHSS) has become a hot research topic. Wire arc additive manufacturing (WAAM) shows remarkable potential for fabricating large LUHSS components; however, repetitive thermal cycles can trigger in situ tempering, significantly affecting carbide evolution and mechanical properties. To address this, this study employed a cold metal transfer plus pulse (CMT+P)-based WAAM process to fabricate thin-walled LUHSS specimens under gradient heat input conditions (3.73, 7.37, and 11.8 kJ/cm) and systematically characterized their mechanical properties and microstructure. The results indicate that increasing heat input enlarges the melt pool and weld bead dimensions, while excessive heat input exacerbates oxide inclusion precipitation and compromises forming quality; the deposited microstructure consists primarily of lamellar martensite, retained austenite, and dispersed carbides. Heat input can precisely control the in situ tempering effect, driving the gradual transformation of carbides from pure Cr23C6 to a coexistence of Cr23C6 and Cr3C, ultimately forming Cr7C3. Grain coarsening and dislocation recovery weaken the material’s strengthening effect. At a heat input of 3.73 kJ/cm, the specimen exhibited excellent strength–ductility synergy and near-isotropic characteristics, with tensile strength, yield strength, and elongation of 1,529 MPa, 1,272 MPa, and 12.3%, respectively. This study shows how heat input-induced in situ tempering regulates precipitation behavior and the mechanical properties of WAAM-formed LUHSS.